Coating applied electrostatically to an object to be coated can be either in the form of electrostatically charged solid particles, i.e., powder, or electrostatically charged liquid particles which have been atomized using a variety of well known techniques or principles, including air impingement atomization, airless or hydrostatic pressure atomization, and/or electrostatic atomization. This invention is useful with both liquid and powder spray coating applications.
In the application of solid particulate coatings, such as powdered resins, in industrial finishing applications, the particulate or powder is commonly conveyed to a spray device, often termed a "gun", by air under pressure and is then sprayed from an opening in the forward end, or nozzle, of the gun in the form of a powder-entrained air stream which is projected along a path from the gun toward the object to be coated. In the process of spraying the coating particles from the gun, an electrical charge is preferably imparted to the particles by an electrode maintained at a high voltage which is mounted to the gun nozzle proximate to the path of the powder coating stream. The charged particles are then electrostatically attracted toward the object to be coated which is held at electrical ground potential, enhancing the efficiency with which charged particles sprayed from the gun are deposited on the target article. After the article is coated, it is generally conveyed through an oven where the powder coating material is heated and fused onto the surface of the article to permanently bond it thereto.
Electrostatic powder spray guns typically include a mechanical powder deflector mounted at the nozzle end of the gun. In one preferred form the deflector is in the shape of a cone and is disposed axially in the flow path of the powder being sprayed from the gun, deflecting the powder into a conical spray pattern. That is, the deflector is impacted by the powder coating material being sprayed from the gun in the nozzle region and directs the powder radially outwardly to form a conical spray pattern.
Electrostatic liquid spray gun systems customarily include a source of pressurized liquid which conveys the liquid coating to the gun via a hose where it is emitted from the nozzle in a stream of atomized particles. Atomization can be produced by impingement of the liquid stream with air in the region of the nozzle, which is known as air atomization. Alternatively, the liquid coating can be highly pressurized such that upon exit from the nozzle atomization results, which is termed hydrostatic or airless atomization. In still other systems, the liquid is subjected to electrostatic forces which effectively atomize the liquid.
One of the objectives in the design of an electrostatic spray gun, either liquid or powder, is to maximize the efficiency with which charged coating particles sprayed from the gun are deposited on the article being coated. This is called the "transfer efficiency". It is generally believed by those skilled in the art that transfer efficiency can be increased by increasing the charge on the particles and/or by increasing the strength of the electrostatic field between the gun and the article being coated.